Method of manufacturing a ceramic, polycrystalline, magnetically anisotropic spinel ferrite body

ABSTRACT

The manufacture of a ceramic, polycrystalline, magnetically anisotropic spinel ferrite bodies by sintering a mixture containing crystals of a compound PbMe&#39;&#39;IIFe16IIIO27 and/or of the compound PbFe12III019, the magnetic preferential axes or magnetic preferential planes of said crystals in said mixture being, to a certain extent, oriented mutually parallel, in which the abovementioned spinel ferrite bodies are formed as the product of a topotactical reaction which mainly occurs according to the equation PbMe2&#39;&#39;IIFe16IIIO27+6MeIIO 6MeIIFe2IIIO4+2Me&#39;&#39;IIFe2III04+PbO and/or according to the equation PbFe12III019+6MeII0 6MeIIFe2III04+Pb0. WHEREIN MeII and Me&#39;&#39;II both represent at least one (possibly all the same) representative of the group formed by the bivalent elements FeII, NiII, MnII, CoII, Zn, Mg, CuII and the bivalent combination (LiI+FeIII)/2 and wherein FeIII may be partly replaced by A1 and/or CrIII and/or at least one of the trivalent combinations (MeII+TiIV(/2 and (MeII+SnIV

United States Patent Ferdinand Clemens Maria Driessens Nijmegen;

Henricus Francisous Johannes Ignatius Giller, Emmasingel, Eindhoven;Dirk Veeneman, Emmasingel, Eindhoven, all of [72] Inventors Netherlands[21] Appl. No. 856,187 [22] Filed Sept. 8, 1969 [45] Patented Dec. 7,1971 [73] Assignee U.S. Philips Corporation New York, NY. [32} PrioritySept. 7, 1968 [3 3] Netherlands [3 l 6812815 [52] user 252/6258,

51 mu ..C0lg49/00,

' c041 35/20 [50] FieldofSearch 252/6256,

[56] References Cited UNITED STATES PATENTS 3,015,788 1/1962 Sirvetz252/6256 3,072,575 1/1963 Gorter et al. 252/6263 OTHER REFERENCES Taft,Alignment Techniques and Microwave Properties of Oriented FerrimagneticCubic Material," Bulletin of American Ceramic Society, Vol. 47, No. 4,page 392 April 1968.

Primary Examiner-Robert D. Edmonds Anorney Frank R. Trifari ABSTRACT:The manufacture of a ceramic, polycrystalline, magnetically anisotropicspinel ferrite bodies by sintering a mixture containing crystals of acompound IbMe"'Fe 'O and/or of the compound PbFe, "'0, the magneticpreferential axes or magnetic preferential planes of said crystals insaid mixture being, to a certain extent, oriented mutually parallel, inwhich the above-mentioned spinel ferrite bodies are formed as theproduct of a topotactical reaction which mainly occurs according to theequation PbMe,'AIIFe, "'O,,+6Me"O 6Me"Fe,'"O,+2Me"'Fe 'O,+PbO and/oraccording to the equation PbFe,,'"0,,+6Me"0 6Me"Fe '"0,+Pb0. wherein Me"and Me" both represent at least one (possibly all the same)representative of the group formed by the bivalent elements Fe", Ni",Mn", Co", Zn, Mg, Cu and the bivalent combination (Li'+ Fe'")/2 andwherein Fe'" may be partly replaced by Al and/or Cr'" and/or at leastone of the trivalent combinations (Me"+Ti")/2 and (Me"+Sn")/2.

METHOD OF MANUFACTURING A CERAMIC, POLYCRYSTALLINE, MAGNETICALLYANISOTROPIC SPINEL FERRITE BODY The invention relates to a method ofmanufacturing a ceramic, polycrystalline, magnetically anisotropicspinel ferrite body and to a spinel ferrite body manufactured by thismethod.

As is known, spinel ferrites have long been used extensively as magneticcore materials in electronic high-frequency apparatus. The term spinelferrites" refers to the materials that show the same (cubic) crystalstructure as the mineral spinel, MgAl Very well-known spinel ferrites inthe scope of the just-mentioned field of application are, for example,the nickel zinc ferrites (built up from mixed crystals of nickelferrite, NiFe O,,, and zinc ferrite, ZnFe O and the manganese zincferrites (build up from mixed crystals of manganese ferrite, MnFe,O.,,and zinc ferrite). For some applications it is desirable to havepolycrystalline spinel ferrite bodies available with a certain texture,"that is to say, polycrystalline spinel ferrite bodies in which themicrocrystals are oriented mutually in a special manner. Such bodies aremagnetically anistropic.

Polycrystalline, magnetically anistropic nickel ferrite bodies werealready described by G. P. Rodnique and L. A. Crouch in an articleentitled Linewidth Narrowing through Grain Orientation of CubicFerrites, see Journal of applied Physics, 37,pp. 923-925 (1966). Thesebodies were manufactured by compressing finely divided nickel ferrite,NiFe O in a constant magnetic field which was oriented alternately inone of the other of two directions mutually enclosing an angle of 70.They showed only a week magnetic orientation. Further reference is madeto a statement Alignment Techniques and Microwave Properties of OrientedFerrimagnetic Cubic Materials (D.R. Lapt, Bulletin American CeramicSociety,47 (4), p. 392 (1968)). From this statement it appears thatmagnetically oriented, polycrystalline bodies obtained by the author andconsisting of, for example, lithium ferrite, nickel ferrite, nickelcobalt ferrite and magnesium-manganese ferrite, in comparison with thecorresponding magnetically unoriented bodies, show a reduced line widthof the magnetic resonance, a reduced resonant field strength and abetter rectangularity characteristic of the hysteresis loop.

Furthermore it was known already to manufacture polycrystalline,ferromagnetic bodies (including spinel ferrite bodies) with mutuallyoriented crystals by converting mutually magnetically oriented grains ofa crystallized ferrite compound with a high axial magnetic crystalanisotropy with nonoriented grains of nonmagnetic compounds in aso-called topotacticalreaction (see F. K. Lotgering, Topotacticalreactions with ferrimagnetic oxides having hexagonal crystal structures,l, Journal of Inorganic and Nuclear Chemistry, 9, pp. 1 13-123, Feb.1959, and French Patent Specification No. 1,203,863, particularly theexamples 2] and 22). However, this method of manufacturing also has sofar had its objections and/or restrictions. When proceeding, forexample, according to the just-mentioned example 21 of the French PatentSpecification No. 1,203,863, according to which a finely divided mixtureof l gram-molecule of BaCo Fe,,,O (so-called barium-cobalt-W") andgram-molecules of cobalt carbonate, CoCO is compressed in a rotatingmagnetic field at right angles to the direction of compression and isthen sintered by heating at a temperature of l,250 C., a polycrystallinebody is formed during sintering according to the reaction.

which body contains bariumferrite, BaFe O, as an impurity in addition tothe mutually magnetically oriented crystals of cobalt ferrite, CoFe O,.This impurity, an (undesirable) dilution of the active magnetic phase,cannot be removed from the sintered body without even more disturbingthe texture thereof which is little pronounced as it is.

According to the prescription given in example 22 of the above-mentionedFrench Patent Specification No. 1,203,863 a finely divided mixture of 2gram-molecules of cobalt ferrite,

CoFe O l gram-molecule of zinc oxide, ZnO, and l grammolecule of ironoxide Fe O is compressed in a linear DC magnetic field parallel to thedirection of compression and then sintered by heating at a temperatureof l,300 C. A polycrystalline cobalt-zinc ferrite body is formed duringthe sint ing according to the reaction equation.

2C0Fe O +Z- nO+Fe O fi 3CO Zn 33Fe2O4 The cobalt ferrite crystals formthe magnetically orientable component of the starting mixture. As isknown, these crystals have a comparatively high magnetic crystalanisotropy as compared with other spinel ferrite crystals. in practice,there'- fore, the field of application of the prescription according tothe above-mentioned example 22 of the French Patent Specification No.1,203,863 is restricted to the manufacture of polycrystalline bodiesconsisting of cobalt ferrite or of cobalt mixed ferrites having a ratherhigh cobalt content.

The invention provides a method which enables the manufacture ofpolycrystalline, magnetically anisotropic spinel ferrite bodies ofmutually very diverging compositions free from a contaminating secondphase, while using the principle of the above-mentioned topotactical"reaction. This method, in which is known manner start is made from amixture which contains as one of the components of crystallized ferritecompound having axial or planar magnetic crystal anisotropy and in whichthe other component of the starting mixture is an oxide having achemical composition according to the formula Me"O, wherein Me" is atleast one representative of the group formed by the bivalent elementsFe", Ni", Mn", Co", Zn, Mg, Cu" and the bivalent combination (Li'+Fe',/2which starting mixture is given the desired shape in such manner thatthe individual crystals of the ferrite compound having axial or planarcrystals anisotropy are to a certain extent oriented parallel mutuallywith their easy axes of magnetization, after which the starting mixturewhich has thus been brought to the desired shape is sintered, ischaracterized in that the crystallized ferrite compound having axial orplanar crystal anisotropy has a chemical composition according to theformula,

2 3).r( )(x-fih wherein x is an even number exceeding 4 and smaller than10 and Me'" also is at least one representative of the group formed bythe bivalent elements Fe", Ni", Mn", Co", Zn, Mg, Cu" and the bivalentcombination (Li'+Fe')/2 When using the method according to the inventionthe formed ferrite compound is stable at the sintering temperature. Leadmonoxide, PhD, is formed as a byproduct of the topotactical reaction andis liberated during sintering in vapor form and escapes. Surprisingly,the desired texture of the formed sintered bodies is not disturbed bythe escape of the lead monoxide so that these bodies, in accordance withthe degree of orientation of the crystal particles having axial orplanar magnetic crystal anisotropy in the starting mixture, show a moreor less strong magnetic anisotropy with the associated above-mentionedphysical properties which are favorable for particular uses.

The starting mixture is preferably brought in the desired form ofcompressing it in a magnetic orienting field. However, the startingmixture, after the addition of a binder, may also be processed intoplates, sheets, leaves, or strips by rolling or extrusion. Actually, theferrite crystals having axial or planar magnetic crystal anisotropy havea strongly developed boundary surface at right angles to the directionof the easy axis of magnetization or parallel to the magnetic preferredplane so that the mutual parallel orientation, to some extent, of theeasy axes of magnetization of the crystals can also be realized by meansof mechanical forces instead of by means of magnetic forces, as is knownper se, for example, from US. Pat. No. 2,999,275. The plates, sheets,leaves, strips obtained by rolling are then sintered after burning thebinder.

According to the invention, very good results are obtained particularlyby starting from a mixture in which the crystallized ferrite compoundhaving axial or planar magnetic crystal anisotropy has a chemicalcomposition according to the formula wherein the symbol Me' again hasthe above-mentioned meaning. The use of the class of compounds definedby this formula (so-called lead-W-compounds) as components of thestarting mixture when using the method according to the inventionpermits obtaining a variety of polycrystalline magnetically anisotropicspinel ferrite bodies of a variety of chemical compositions in a simplemanner and in a pure state.

Without departing from the scope of this invention, the Fe' in thecrystallized ferrite compound having axial or planar magnetic crystalanisotropy may be replaced partly by at least one representative of thegroup formed by the trivalent elements A1, and Cr and the trivalentcombination (Me"+Me )/2 wherein Me" has the abovedefined meaning and Meis at least one of the elements Ti and Sn Furthermore, the othercomponent of the starting mixture, i.e. the oxide having a chemicalcomposition according to formula Me"O, may be replaced fully or partlyby one or more compounds which are converted into an oxide Me" uponheating. As such components, for example, may be mentioned carbonates.The two Just-mentioned types of replacements are generally known inanother connection and hence form in themselves no part of the presentinvention.

Monocrystals of ferromagnetic spinel ferrites have four mutuallyequivalent easy axes of magnetization in the four directions at rightangles to the planes of a regular octahedron, one of the basic forms ofthe cubic (or regular") crystal system. The crystallographic indicationfor these directions is lll" or hhh. By means of X-ray diffractionexamination it can be established to what extent a certain texture ispresent in a given polycrystalline spinel ferrite body. For that purposean X-ray diffraction pattern of the spinal ferrite body in question iscompared with that of a textureless spinel ferrite body of the samecomposition and build up from the same type of crystals. In both casesa. the intensities of the four hhh reflections;

the intensities of all reflections (hkl reflections) including the hhhreflections mentioned sub (a) are summed.

The sumof the intensities mentioned sub (a) are denoted by the symbol 21those of the intensities mentioned sub (b) are denoted by the symbol ElIntroduced are now the quantities p and p, which are defined as follows:

As a measure of the texture of a given spinel ferrite body serves theso-called directional factor, f, which is defined by the relationship 3for the body to be tested for texture;

, for a texture-less body.

EXAMPLE I A mixture of PbFe O and NiO in a mutual ratio of 1 mol PbFe Oand 6 mol MO was ground in a vibrating mill with acetone for 2 hours.The component PbFe O was present in this mixture in the form ofmagnetically orientable crystals having a magnetic preferred directionparallel to the hexagonal c-axis. The mixture was sieved while wet and apart of the formed slurry was compressed to a cylindrical tablet havinga diameter of 35 mm. and a height of 15 mm. in a constant DC magneticfield having a field strength of 4,000 oersted parallel to the directionof compression at a pressure of 0.5 ton/sq.cm. (a DC magnetic field isto be understood to mean herein a magnetic field of which neither thedirection nor the intensity varies). The tablet was dried,after-compressed isostatically at a pressure of 1,000 atmospheres, andheated for 4 hours at l,250 C. in flowing oxygen which was supplied at arate of 2 liters per minute. The course of the reaction occurring may berepresented by the equation The gas current was sufficiently large todissipate the PbO which evaporated during the reaction and to catch itin the cold part of the furnace. An X-ray diffraction pattern was madeof the sintered tablet of a plane which extended at right angles to thedirection in the tablet in which during compression, as a step of themanufacture of the tablet, compression was carried out. This directionin the tablet will hereinafter briefly be referred to as direction ofcompression.

ln substantially the same manner another tablet of nickel ferrite wasprepared with the difference that during the preparation the treatmentwith a magnetic field was omitted. Of this tablet also an X-raydiffraction pattern was made of a plane at right angles to the directionof compression. By comparing the two patterns a value of 0.8 for thedirectional factor, f, of the first-mentioned tablet was found in themanner described above.

EXAMPLE ll Two tablets of the compound Ni Mn .,Fe-,0 were prepared froma mixture of PbFe O M0, and MnCO in a mutual ratio of 1 mol PbFe 0 3.6mol Nio and 2.4 mol MnCo in the manner described in example I by heatingthe mixture for 6 hours at a temperature of 1,320 C. in flowing nitrogenwhich was supplied at a rate of 2 liters per minute. The course of thereaction occurring may be represented by the equation An X-raydiffraction pattern was made of each of the two tablets of a face atright angles to the direction of compression. By comparing the twopatterns, a value of 0.6 was found for the directional factor, f, of thetablet during the manufacture of which a magnetic field was used duringcompression (with a field strength of 4,000 oersted).

EXAMPLE Ill Two tablets of the compound Ni Co Fe 0, were prepared in the'manner described in example I from a mixture of PbFe O MO, and CoCO ina mutual ratio of 1 mol PbFe O 5.4 mol NiO and 0.6 mol CoCO by heatingthe mixture for 6 hours at a temperature of 1,260" C. in flowing oxygenwhich was supplied at a ratio of 2 liters per minute. The course of thereaction occurring may be represented by the equation From thecomparison of the two X-ray diffraction patterns it was found that thetablet during the manufacture of which a constant DC magnetic field(with a field strength of 4,000 oersted) was used during compression,showed a directional factor, f, of 0.5.

EXAMPLE IV In substantially the same manner as described in example Ill,two tablets of the same composition were made in which the mode ofpreparation differed from that described in example III in that thistime heating was carried out for 6 hours and at 1,320" C. in flowingnitrogen which was supplied at a rate of 2 liters per minute. This timea directional factor, f, of 0.8 was found for the tablet in which atexture is produced by the magnetic field treatment.

EXAMPLE v Two tablets of the compound Ni ,,Mg,, ,Fe O were made in themanner described in example I from a mixture of PbFe O MO, and MgO in amutual ratio of 1 mol PbF m w, 5

From the comparison of the two X-ray patterns it was found that thetablet during the manufacture of which a constant DC magnetic field(with a field strength of 4,000 oersted) was used during compression,showed a directional factor, f, of 0.8.

EXAMPLE Vl Two tablets of the compound (Ni ,Co Fe Q were made in themanner described in example I from a mixture of PbFe O MO, and C0 0 in amutual ratio of 1 mol PbFe O 5.4 mol NiO, and 0.0606 mol C0 0. byheating the mixture at a temperature of l,300 C. for 4 hours in flowingoxygen which was supplied at a rate of 2 liters per minute. The courseof the reaction occurring may be represented by the equation For thetablet in which a texture was produced by the magnetic field treatment,a directional factor, f, of 0.8 was found in this case.

EXAMPLE VI! A mixture of PbCQ Fe O- and MO in a mutual ratio of 1 molPbCo Fe o and 6 mol MO was ground in a vibrating mill for 2 hours withacetone. The component PbCo Fe, O was present in this mixture inthe formof magnetically orientable crystals having a magnetic preferred plane atright angles to the hexagonal c-axis. The mixture was sieved while wetand a part of the slurry formed was compressed to a cylindrical tablethaving a diameter of 35 mm. and a height of mm. in a rotating magneticfield of 3 rotations per second and which extended at right angles tothe direction of compression. The rotating magnetic field had a constantfield strength of 3,600 oersted. The tablet was dried, after-compressedisostatically at a pressure of 1,000 atmospheres and heated at 1,250" C.for 6 hours in flowing oxygen which was supplied at a rate of 2 litersper minute. The course of the reaction occurring may be represented bythe equation Another tablet was made from the same slurry but withoutthe use of a magnetic field. An X-ray difiraction pattern was made ofeach of the tablets of a plane at right angles to the direction ofcompression. By comparing the two patterns a value of 0.9 was found forthe directional factor, f, of the firstmentioned tablet.

EXAMPLE VI" in substantially the same manner as described in example Vlltwo tablets of the compound C0o 25MnoJ5Fe204 were made by heating amixture of 1 mol PbCo Fe o and 6 mol MnCO at a temperature of 1300' C.for 6 hours in flowing oxygen which was supplied at a rate of 2 litersper minute. The course of the reaction occurring may be represented bythe equation An X-ray diffraction pattern was again made of each tabletof a plane at right angles to the direction of compression. By comparingthe two patterns a value of 0.5 was found for the directional factor, f,of the tablet during the manufacture of which the magnetic rotatingfield was used.

EXAMPLE IX.

A mixture of PbNi2Fe 6O21 and CuO in a mutual ratio of 1 mol PbNi Fe Oand 6 mol CuO was ground in a vibrating mill with acetone for 2 hours.The component PbNi Fe O was present in this mixture in the form ofmagnetically orientable crystals having a magnetic preferred directionparallel to the hexagonal c-axis. The mixture was sieved while wet and apart of the resulting slurry was compressed at a pressure of 0.5 ton/sq.cm. to a cylindrical tablet having a diameter of 35 mm. and a height ofIS mm. in a constant DC magnetic field having a field strength of 4,000oersted parallel to the direction of compression. The tablet was driedand after-compressed isostatically at a pressure of l,000 atmospheres.The tablet was then heated for hours at a temperature of l, 1 50 C. inflowing oxygen which was supplied at a rate of 2 liters per minute. Thecourse of the occurring reaction may be represented by the equation Insubstantially entirely the same manner another tablet was manufacturedfrom the same slurry with the difference that during the manufacture ofthis second tablet the treatment with a magnetic field was omitted. Ofthis tablet also an X-ray difiraction pattern was made of a plane atright angles to the direction of compression. By comparing the twopatterns a value of the direction factor, f, of 0.8 was found for thefirst mentioned tablet.

EXAMPLE X In the same manner as described in example lX two tablets wereprepared consisting of the compound Zn Mn Fe O starting from a mixtureof 1 mol PbZn- Fe O and 6 mol Mn- C0,.

During the manufacture of both tablets heating was carried out for 6hours at a temperature of 1,350 C. in flowing nitrogen which wassupplied at a rate of 2 liters per minute. Of course of the reactionoccurring may be represented by the equation By comparing the two X-raydiffraction patterns a value of 0.50 was found for the directionalfactor, f, of the tablet during the manufacture of which the DC magneticfield was used.

EXAMPLE Xl 65.5 parts by weight of a mixture of PbFe O and NiO in amutual ratio of 1 mol PbFe O and 6 mol MO was processed to a rollablemass with 24 parts by weight of a solution (consisting of 1 part byweight of polystyrene and 3 parts by weight of trichloroethane) and 4.8parts by weight of tricresylphosphate. The trichloroethane was thenremoved by drying up the mass. The dried mass was rolled to form a film.This film was folded in the direction of rolling and rolled again whichoperation was repeated until a film with maximum texture was obtained(directional factor, f, between 0.6 and 0.8). A film manufactured inthis manner was slowly heated to a temperature of 900 C. as a result ofwhich the organic binder was burnt. Heating was then carried out for 4hours at a temperature of l,250 C. in flowing oxygen which was suppliedat a rate of 2 liters per minute. A reaction took place, whilesintering, the course of which may be represented by the equationAnother sintered body was manufactured from the same mass withoutrolling operation. An X-ray diffraction pattern was made of thefirst-mentioned sintered body of a plane parallel to the direction ofrolling. An X-ray diffraction pattern was also made of an arbitraryplane of the second sintered body. By comparing the two patterns a valueof the directional factor, f, of 0.3 was found for the first sinteredbody.

What is claimed is:

l. A method of manufacturing a ceramic, polycrystalline, magneticallyanisotropic spinel ferrite body comprising the steps of forming amixture of an oxide having a chemical composition according to theformula Me"O, wherein Me" is at least one member of the group consistingof the bivalent elements Fe, Ni", Mn", Co", Zn, Mg, Cu and the bivalentcombination (Li'+Fe'")/2 and a crystallized ferrite compound havingaxial or planar magnetic crystal anisotropy and a chemical compositionaccording to the formula z a Memo) (1-6). wherein x is an even numberlarger than 4 and smaller than and Me" is at least one member of thegroup consisting of the bivalent elements Fe", Ni", Mn",Co, Zn, Mg, Cuand the bivalent combination (Li+Fe")/2 in proportions forming uponheating a ferrite having a composition Me"Fe O compacting said mixtureinto a desired shape in such a manner that the individual crystals ofthe ferrite compound having axial or planar magnetic crystal anisotropyare oriented mutually in parallel to a certain extent with their easyaxes of magnetization or magnetic preferred planes, and thereaftersintering said mixture at a temperature between about l,150 C. and l,350C. for a time sufi'rcient to form the ferrite body.

2. A method as claimed in claim 1, wherein the mixture is compacted in amagnetic directional field.

3. A method as claimed in claim 1, wherein the mixture is admixed with abinder and rolled or extruded into plates, sheets, leaves, or strips,after which the binder is burned and the plates, sheets, leaves, orstrips are sintered.

4. A method as claimed in claim 1, wherein the crystallized ferritecompound having axial or planar magnetic crystal anisotropy has achemical composition according to the formula Pbo. (Fe2'o3)g( Me llo)g.

5. A method as claimed in 1 wherein the Fe" in the crystallized ferritecompound having axial or planar magnetic crystal anisotropy is partiallyreplaced by at least one member of the group consisting of the trivalentelements A] and Cr'" and the trivalent combinations (Me"+Me"')/2,wherein Me" is defined in claim 1 and Me is at least one of the elementsselected from the group consisting of Ti and Sn.

6. A method as claimed in claim 4 in which the crystallized ferritecompound if PbFe O 7. A method as claimed in claim 4 in which thecrystallized ferrite compound is PbCo Fe O 8. A method as claimed inclaim 4 in which the crystallized ferrite compound is PbNi Fe O 9. Amethod as claimed in claim 4 in which the crystallized ferrite compoundis PbZn Fe,,,O

2. A method as claimed in claim 1, wherein the mixture is compacted in amagnetic directional field.
 3. A method as claimed in claim 1, whereinthe mixture is admixed with a binder and rolled or extruded into plates,sheets, leaves, or strips, after which the binder is burned and theplates, sheets, leaves, or strips are sintered.
 4. A method as claimedin claim 1 wherein the crystallized ferrite compound having axial orplanar magnetic crystal anisotropy has a chemical composition accordingto the formula PbO. (Fe2IIIO3)8(Me''IIO)2.
 5. A method as claimed in 1wherein the FeIII in the crystallized ferrite compound having axial orplanar magnetic crystal anisotropy is partially replaced by at least onemember of the group consisting of the trivalent elements Al and CrIIIand the trivalent combinations (MeII+MeIV)/2, wherein MeII is defined inclaim 1 and MeIV is at least one of the elements selected from the groupconsisting of TiIV and SnIV.
 6. A method as claimed in claim 4 in whichthe crystallized ferrite compound is Pb Fe12O19.
 7. A method as claimedin claim 4 in which the crystallized ferrite compound is PbCo2Fe16O27.8. A method as claimed in claim 4 in which the crystallized ferritecompound is PbNi2Fe16O27.
 9. A method as claimed in claim 4 in which thecrystallized ferrite compound is PbZn2Fe16O27.